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 Post subject: Utility response for data request
PostPosted: Fri Feb 12, 2016 1:56 pm 
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Location: West Central, OH
Attempting to follow the IEEE 1584 guidance for utility data, we received the following response:

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Because of the dynamic nature of the electric transmission and distribution system, Public Service utilizes a standardized “table-driven” approach to provide customers with the information necessary for the customer to calculate arc flash energy within their particular facility. The publication, “Xcel Energy Standard for Electric Installation and Use,” provides tables that list secondary fault-current based on an infinite supply-bus and standard transformer impedances along with transformer over-current-protection information to enable the customer to calculate protection clearing times for their specific facility. This approach permits Public Service to provide technical information to the customer that the customer can use to complete its own arc flash analysis over a range of clearing times and fault magnitudes without the Company driving any sort of engineering modeling or system characteristic assumptions that are the customer’s responsibility to assume.
Public Service is not obligated by any statute, regulation, or generally accepted utility practice to meet the data requirements of IEEE 1584. In fact, that particular standard is only one method for performing arc incident energy calculations. It is not the "only" or "approved" method. The tables in section 410 of the National Electric Safety Code are based on calculations from a software package called “Arc-Pro” which does not use the IEEE 1584 method.
The "infinite bus" secondary fault current is defined as fault current without taking into account any primary or secondary distribution conductor impedance to provide an upper boundary for the arc flash magnitude. The largest factor that drives the fault current impedance is the distribution transformer. Therefore, a qualified consultant will know that the actual fault current experienced in any incident will be somewhat less, but not greatly less, than the "infinite bus" secondary fault current provided in the Public Service tables.
Due to the variability of how an individual customer is served on the distribution system as a result of everyday switching and system construction, estimates of ground resistance, and various other engineering assumptions, a range of fault currents is possible. It is Public Service’s position that the customer and the customer's consultant is in the best position to determine what assumptions are valid in performing the calculations over a range of possibilities.


This is the only utility to respond in this fashion in the over 50 we have contacted. Any guidance for a path forward with the utility or just give up on them?


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 Post subject: Re: Utility response for data request
PostPosted: Sun Feb 14, 2016 12:28 pm 
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willcoc wrote:
Attempting to follow the IEEE 1584 guidance for utility data, we received the following response:

Quote:
Because of the dynamic nature of the electric transmission and distribution system, Public Service utilizes a standardized “table-driven” approach to provide customers with the information necessary for the customer to calculate arc flash energy within their particular facility. The publication, “Xcel Energy Standard for Electric Installation and Use,” provides tables that list secondary fault-current based on an infinite supply-bus and standard transformer impedances along with transformer over-current-protection information to enable the customer to calculate protection clearing times for their specific facility. This approach permits Public Service to provide technical information to the customer that the customer can use to complete its own arc flash analysis over a range of clearing times and fault magnitudes without the Company driving any sort of engineering modeling or system characteristic assumptions that are the customer’s responsibility to assume.
Public Service is not obligated by any statute, regulation, or generally accepted utility practice to meet the data requirements of IEEE 1584. In fact, that particular standard is only one method for performing arc incident energy calculations. It is not the "only" or "approved" method. The tables in section 410 of the National Electric Safety Code are based on calculations from a software package called “Arc-Pro” which does not use the IEEE 1584 method.
The "infinite bus" secondary fault current is defined as fault current without taking into account any primary or secondary distribution conductor impedance to provide an upper boundary for the arc flash magnitude. The largest factor that drives the fault current impedance is the distribution transformer. Therefore, a qualified consultant will know that the actual fault current experienced in any incident will be somewhat less, but not greatly less, than the "infinite bus" secondary fault current provided in the Public Service tables.
Due to the variability of how an individual customer is served on the distribution system as a result of everyday switching and system construction, estimates of ground resistance, and various other engineering assumptions, a range of fault currents is possible. It is Public Service’s position that the customer and the customer's consultant is in the best position to determine what assumptions are valid in performing the calculations over a range of possibilities.


This is the only utility to respond in this fashion in the over 50 we have contacted. Any guidance for a path forward with the utility or just give up on them?


Have fun. It's the most bureacratic-oriented utility I've ever dealt with. Be persistent and you can eventually get to the relaying or maintenance departments that are a lot more helpful.


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 Post subject: Re: Utility response for data request
PostPosted: Mon Feb 15, 2016 8:05 am 
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Some utilities don't have the manpower to keep up with an extensive data base. However, I find it hard to believe this company is one of them. Many members of the forum community have no trouble obtaining good data while others have quite a struggle. Here is what really jumped out at me though. It is the following statement:

"In fact, that particular standard is only one method for performing arc incident energy calculations. It is not the "only" or "approved" method. The tables in section 410 of the National Electric Safety Code are based on calculations from a software package called “Arc-Pro” which does not use the IEEE 1584 method."

Regardless of which method you use, incident energy is primarily a function of two main variables - short circuit current and arc duration (and a few secondary variables like gap). Whether you use IEEE 1584, arc pro, etc. you still need this information so the above statement is silly at best. It would be nice if it was as realistic as possible. This tells me they might be grabbing at straws to justify not providing the data.

About all you can do is a) document what happened, b) use the initial short circuit current data and then gradually reduce it incrementally until you reach a value where overcurrent devices begin to respond in the time delay region and you see incident energy values jump. That at least defines a range of validity. I've mentioned this elsewhere in the forum too. Good Luck!


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 Post subject: Re: Utility response for data request
PostPosted: Mon Feb 15, 2016 9:08 am 
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This is the utility at my front door and that is the official stance - use the tables! Even as large as they are they do not have the man power to supply every request. Even if they did it would only be accurate for that moment and could change very quickly or maybe never. They have several changes over the past several years with the addition of generation, shut-down of plants and modifications to substations that happen everyday within the 100's or 1000's of substations.

I do not like using infinite bus data. I think there are much better ways of handling fault currents and energy results.


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 Post subject: Re: Utility response for data request
PostPosted: Mon Feb 15, 2016 12:08 pm 
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Quote: wrote:
[...]the actual fault current experienced in any incident will be somewhat less, but not greatly less, than the "infinite bus" secondary fault current provided in the Public Service tables.


In case of "infinite bus", a 500KVA 5%Z 13800/480V distribution transformer would limit fault current down to 12KA. In the real world, fault current on secondary side of the 500KVA transformer would be less, and could be radically less. For example, with 5MVA SC source feeding the transformer, the fault current would be reduced down to approx 3KA or by factor of 4.

In an ideal world, utilities provide with the available available fault current value along with margin of error for the value at service entrance and keep their customers updated of any changes to it.


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 Post subject: Re: Utility response for data request
PostPosted: Tue Feb 16, 2016 10:04 am 
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arcad wrote:

In an ideal world, utilities provide with the available available fault current value along with margin of error for the value at service entrance and keep their customers updated of any changes to it.


In my ideal world, utilities do everything possible to keep the lights on; and failing that, do everything possible to get the lights back on. These two ideals require nearly constant re-routing of power through varying impedances, and changing the transformer impedance when a failure occurs at 2:00 AM. Not to sure how our various ideals can be resolved.


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 Post subject: Re: Utility response for data request
PostPosted: Tue Feb 16, 2016 10:32 am 
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Many of the statements made by that utility jump out at me also. For instance:

The "infinite bus" secondary fault current is defined as fault current without taking into account any primary or secondary distribution conductor impedance to provide an upper boundary for the arc flash magnitude.

The infinite bus may not provide an upper boundary for the arc flash magnitude but more likely the lower boundary as the infinite bus current may cause the protective devices to trip in the instantaneous region whereas the actual short circuit may cause the protective devices to trip in the short time or long time region which may result in higher arc flash magnitudes.

Overall, reading the utilities statement makes me wonder if they really understand arc flash.

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 Post subject: Re: Utility response for data request
PostPosted: Tue Feb 16, 2016 12:03 pm 
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stevenal wrote:
In my ideal world, utilities do everything possible to keep the lights on; and failing that, do everything possible to get the lights back on. These two ideals require nearly constant re-routing of power through varying impedances, and changing the transformer impedance when a failure occurs at 2:00 AM. Not to sure how our various ideals can be resolved.


I see no conflict between our views. I was trying to make a point that the assumption of infinite bus may result in unrealisticaly high predicted short circuit fault currents. Utilities should be able and normally do provide short circuit current and fault power factor values at service entrance. The amount of fault current would usually decrease during system failure and consequent power-rerouting, and it would come up back to its steady value within short time interval once the system has been restored. If the analysis is done based on IEEE 1584 empirical model, the model requirement for making calculations at arcing current reduced by 15% should take care of such case scenario.


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 Post subject: Re: Utility response for data request
PostPosted: Tue Feb 16, 2016 6:18 pm 
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arcad wrote:

I see no conflict between our views. I was trying to make a point that the assumption of infinite bus may result in unrealisticaly high predicted short circuit fault currents. Utilities should be able and normally do provide short circuit current and fault power factor values at service entrance. The amount of fault current would usually decrease during system failure and consequent power-rerouting, and it would come up back to its steady value within short time interval once the system has been restored. If the analysis is done based on IEEE 1584 empirical model, the model requirement for making calculations at arcing current reduced by 15% should take care of such case scenario.


So I tried a quick simulation requiring no rerouting, just a very simple reconfiguration. A distribution substation with two transformers normally run in parallel needs maintenance or suffers an event so that one of the two transformers is taken out of service. The resulting available current at the terminals of a 480 V three phase transformer fed from this substation drops to 83% of its normal value. We certainly have done much more creative reconfigurations, sometimes using line regulators to keep the voltage acceptable while routing much longer distances in order to take a substation completely off-line. A few times we've even used the distribution to back feed a substation transformer connected to an isolated piece of transmission in order to feed another substation. And the entity that supplies us also is charged with keeping the power flowing, and plans maintenance during the same low load season. I don't think 15% will cover these situations. Suggest using 50% instead.


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 Post subject: Re: Utility response for data request
PostPosted: Mon Feb 22, 2016 9:36 am 
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We obtained the phone number for the person at the utility who fields commission complaint issues. He listened but indicated he was not an engineer and did not fully understand but would check in the current policy and see what could be done. Will provide an update when we have additional information from the utility.


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 Post subject: Re: Utility response for data request
PostPosted: Mon Feb 22, 2016 10:55 am 
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https://www.xcelenergy.com/staticfiles/xe/Corporate/Corporate%20PDFs/XcelEnergy_Standard_For_Electric_Installation_and_Use2014.pdf

See page 58.


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 Post subject: Re: Utility response for data request
PostPosted: Mon Mar 07, 2016 4:41 am 
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We where able to receive a "technical response from the utility company. I have left the names in since it was already in another post.
Quote:
I received a call this morning from an XCEL engineer, who was really quite helpful. He explained to me that he would LOVE to give me whatever fault information XCEL energy had, IF they had it. He explained that the company had no computer model of its electrical system, and providing fault information is always a manual affair. For that reason alone, XCEL doesn’t provide fault data on its system. They simply don’t have it. Honestly, I was amazed, and intensely sympathetic towards the XCEL engineers.

That revelation aside, he explained that he would be quite willing to provide system data on their distribution network that would be sufficient for me to add it to OUR model, and get a good estimate of actual fault back through their system. I should get something from him in the next day or so.


I wanted to close the loop on this posting so that others know that at times, it takes time and many phone calls but the enough data can be obtained so that the wonderful term of "infinite bus" does not need to be used.


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 Post subject: Re: Utility response for data request
PostPosted: Mon Mar 07, 2016 5:47 am 
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I am really surprised that a company that big does not have a computer model of their system. Even here in VT, except for the smallest utilities, they have computer models of their system. The smallest one that does here is only 10,000 customers.

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